Due to the heterogeneity of subterranean formations, the acid fluids used in acidizing treatments enter the most permeable intervals which have the least resistance to flow, thereby increasing the permeability of those intervals and causing them to accept ever more acid. In order to improve acid displacement and prevent this uneven distribution of the acid, the acid must be diverted from most permeable intervals of the targeted formation into the less permeable or damaged intervals. In order to distribute the acid fluid evenly along the whole formation, generally one stage of acid fluid is pumped into the formation followed by a diverting agent stage (a diverting agent in the acid fluid) to temporarily plug off an interval of the formation where the previous stage acid fluid penetrated. Subsequently, the following stage acid fluid is forced into the other intervals of the formation. It is often important to use diverting agents for acid stimulation of long horizontal well bores.
Several materials have been utilized as diverting agents in acid stimulation treatments in the past. Oil-soluble naphthalene particles have been used as diverting agents. The problem with using oil-soluble diverting agents is that the chance of the diverting agents contacting oil produced from stimulated formations is very rare. The reacted acid fluid is physically in between the oil-soluble diverting agents and the formation oil. Foam has also been used as acid diverting agents, but it is very difficult to control the size distribution of the foams in downhole conditions for different permeability formations.
Other materials used as diverting agents in well bore treatments in the past have included graded phthalimide particles, polyester polymers, solid azo compounds having an azo component and a methylenic component, low molecular weight condensation products of hydroxyacetic acid with itself or with compounds containing other hydroxyl-, carboxylic-acid- or hydroxycarboxylic-acid moieties, and dispersions of water swellable crosslinked polymer particles. However, various problems exist in using these materials as acid diverting agents. The desired diverting materials will be stable in acid fluid and be easily removed from the formation after the treatment.
It would be desirable if a composition and method could be devised to overcome some of the problems in the conventional acid fluid diverting agents.